1. Field of the Invention
The present invention relates to a method of detecting the presence of target bio-molecules or a concentration of the target bio-molecules using a field effect transistor, and more particularly to a method of detecting the presence of the target bio-molecules or the concentration of the target bio-molecules using the field effect transistor without fixing a probe bio-molecule on a gate sensing surface of the field effect transistor.
2. Description of the Related Art
A transistor based bio-sensor which includes a transistor is one type of sensor which detects bio-molecules using electric signals. Since a semiconductor is used to manufacture the transistor based bio-sensor, the cost of manufacture and the amount of time required to detect the bio-molecules using the electric signals are reduced. Accordingly, much research on this type of sensor has been carried out.
U.S. Pat. No. 4,238,757 discloses a field effect transistor (“FET”) which can be used to detect biological reactions. Using the FET, a bio-sensor measures a change in current of an inversion layer of a semiconductor resulting from changes in the surface charge concentration in order to detect an antigen-antibody reaction. Employing a bio-sensor using the FET, a protein among bio-molecules can be detected. U.S. Pat. No. 4,777,019 discloses a sensor for measuring hybridization of biological monomers with complementary monomers by adsorbing the biological monomers onto a surface of a gate using a FET.
U.S. Pat. No. 5,846,708 discloses a method of determining a presence of hybridization by an extinction of coupled bio-molecules using a charged couple device (“CCD”). U.S. Pat. Nos. 5,466,348 and 6,203,981 disclose a method of increasing signal to noise ratio (“SNR”) using a thin film transistor (“TFT”) with a circuit.
The use of the FET as a bio-sensor decreases costs and reduces the amount of time required to detect the bio-molecules, and the FET is easily used together with an integrated circuit/microelectromechanical system (“IC”)/(“MEMS”).
FIG. 1A is a schematic front view illustration of a structure of a conventional FET sensor of the prior art. Referring to FIG. 1A, the FET includes a substrate 11 doped with an n-type or a p-type material, a source 12a and a drain 12b which are formed apart from each other on two sides of the substrate 11 and the source 12a and the drain 12b are each doped having an opposite polarity to the substrate 11 and a gate 13 formed on the substrate 11 which contacts the source 12a and the drain 12b. Generally, the gate 13 includes an oxide layer 14, a poly silicon layer 15 and a gate electrode 16. A channel is generally formed between the source 12a and the drain 12b. Probe bio-molecules 18 are adhered to the sensing surface of the gate electrode 16 which faces a reference electrode 17. The probe bio-molecule 18 binds to a target bio-molecule (not shown) through a hydrogen bond, or the like, and the bond is detected using an electrical method.
FIG. 1B is a schematic front view illustration of a process of immobilizing probe bio-molecules 18 on the surface of a gate electrode 16 of the FET illustrated in FIG. 1A and binding target bio-molecules (not shown) with the probe bio-molecules 18. Referring to FIG. 1B, a current flowing through a channel varies according to the presence of the immobilized probe bio-molecules 18 on the surface of the gate electrode 16 and the presence of the bond between immobilized probe bio-molecules 18 and the target bio-molecules (not shown), and thus the target bio-molecules can be detected by measuring a variance in the current flowing through the channel.
In all conventional FET structures, probe bio-molecules such as an oligonucleotide or a polymerase chain reaction (“PCR”) product are immobilized on the surface of a gate electrode. An immobilizing technology is used to manufacture a microarray or a modified technology is used to immobilize the bio-molecules. In International Publication No. WO 03/062811, for example, a surface of a gate is treated with a poly-L-lysine (“PLL”) having a positive charge using a wet process, deoxyribonucleic acid (“DNA”) is spotted on the surface of the gate using a spotter and a voltage is measured before and after the spotting.
However, a FET including probe bio-molecules immobilized on the surface of a gate should be disposed of after use and a response time of the sensor is not fast enough. Further, an additional process, such as a coating or depositing an additional layer, is required to immobilize the probe bio-molecules. However, it is expected that this additional processing will vary characteristic properties between the FETs. In addition, it is difficult to use spotting to immobilize the probe bio-molecules on a lab-on-a-chip (“LOC”).